1. Field of the Invention
The present invention relates to a bicycle derailleur, and more particularly to a bicycle rear derailleur.
2. Description of Related Art
A rear derailleur is mounted on a rear fork of a bicycle and is connected with a derailleur lever or a derailleur sleeve mounted on a handle of the bicycle to switch the chain between sprocket wheels on the bicycle. A conventional rear derailleur substantially comprises a mounting base, a moveable base, and a chain guide. The mounting base is mounted on a frame of the bicycle. The moveable base is connected to the mounting base by a linkage assembly. The chain guide is connected to the mounting base. When the moveable base is moved relative to the mounting base, the chain guide can switch the chain between the sprocket wheels so as to change the gear of the bicycle. For example, U.S. Pat. Nos. 7,674,198 and 7,871,346 disclose conventional rear derailleur.
The chain guide is swingable relative to the mounting base and provides a pre-torsion in counterclockwise to a spring. Accordingly, when the chain is switched to a sprocket wheel having a small diameter from a sprocket wheel having a large diameter, the chain can be tightened to prevent the chain from escaping from the sprocket wheels. However, when the bicycle is riding in a rugged road condition, the chain guide may be rotated in counterclockwise and the chain may be loosened. To prevent the chain guide from being loosened, a rear derailleur with rotation resistance is provided, for example US Patent Publication No. 2009/0054183, U.S. Pat. No. 8,852,041, U.S. Pat. No. 8,870,692, or China Patent Publication No. 104554606A.
In US Patent Publication No. 2009/0054183, a friction assembly is mounted on a moveable base to provide a friction to a chain guide. The friction assembly substantially comprises a friction unit, a unidirectional clutch, and an adjusting unit. The friction comprises a first connection board and a second connection board connected by multiple friction boards. When the chain guide that is connected to the mounting base is rotated in counterclockwise, the friction assembly can provide friction to the chain guide to keep the chain in a stable condition. However, the fiction assembly of the '183 Patent is complicated in structure and is difficult to assembled, and the friction provided by the friction assembly cannot be adjusted.
With reference to FIG. 12, in U.S. Pat. No. 8,852,041 and U.S. Pat. No. 8,870,692, a friction assembly 91 is mounted in a moveable base 90, and the friction assembly 91 comprises a rotatable unidirectional clutch 92 and a friction unit 93. The unidirectional clutch 92 is connected to a rotating axle driven by a chain guide and provides a friction to the chain guide in a pre-determined direction. The friction unit 93 has a Ω shape and a circular friction applying segment mounted around the unidirectional clutch 92 to provide a friction to a rotation of the unidirectional clutch 92. However, the large space is necessary for assembling the friction assembly 91, so the volume of the mounting base 90 has to be enlarged. In addition, the circular friction applying segment cannot provide a stable friction to the unidirectional clutch 92, and the friction provided by the friction assembly 91 cannot be adjusted.
In China Patent Publication No. 104554606A, a friction assembly is mounted in a mounting base, and the friction assembly comprises a friction unit and a clutch assembly. The clutch assembly comprises a first clutch and a second clutch. The first clutch is connected with an axle driven by a chain guide in a torque-transmitting manner. The second clutch has an axial cylindrical protrusion. The friction unit comprises a circular segment mounted around the axial cylindrical protrusion of the second clutch and two free ends. The friction unit can provide a friction to the rotation of the clutch assembly, and the friction can also be applied to the rotation of the chain guide. However, the friction unit of the '606 Patent has a Ω space, and a large space for assembling the friction assembly is necessary. The friction provided by the circular segment of the friction unit is unstable, and the friction provided by the friction assembly is not adjustable.
To overcome the shortcomings, the present invention tends to provide a rear derailleur to mitigate or obviate the aforementioned problems.